1. Field of the Invention
The invention relates to apparatus and method for producing fuel gases from carbonaceous matter. In particular, the invention relates to coal gasification by plasma arc torch pyrolysis.
2. Description of the Prior Art
It is well known that the finding rate of natural gas and oil in the Western World has greatly decreased in recent years while the demand has steadily risen. As a result, the United States has become increasingly dependent on foreign sources to meet its gas and oil demands. Recently, it has been estimated by the Institute of Gas Technology that the demand for natural gas in the United States will exceed production in the United States (including imports from Mexico and Canada) by 7.8 trillion cubic feet in 1980 and 18.3 trillion cubic feet in 1990 unless some new means can be found to supplement the supply.
In order to assure the energy independence of the United States, there is an acute need to develop a new source of clean fuel to meet the energy demand. In the United States, coal and oil shale are the only remaining fossil fuel sources which are abundantly available. Numerous attempts have been made to develop a workable process for coal gasification. However, to date there is no known process which can satisfactorily convert the energy of virtually any type of coal into a pipeline quality gas.
The basic requirement for coal gasification includes heating the coal to reaction temperature in the presence of selected reactants to induce certain chemical reactions. The combination of coal, heat and a reactant, such as water, produces raw gas which is essentially hydrogen, carbon monoxide and methane. The raw gas can be further shifted and beneficiated by conventional means to produce a gas which is essentially methane.
As stated in the May 1973 issue of "Pipeline and Gas Journal" at pages 29-31, the conventional coal gasification processes utilize a portion of the input coal for burning to generate the heat required by the process. That is, the endothermic heat is applied by injecting into the reaction chamber enough air or pure oxygen to cause combustion of part of the coal. Normally, between 14% to 26% of the coal charged into the process is burned to supply the endothermic heat requirement. This portion of the carbon is essentially "lost" to the process, since it forms carbon dioxide which has no heating value. The use of pure oxygen to cause combustion results in a raw gas which is diluted by carbon dioxide. Combustion with air further dilutes the outgas with nitrogen to a level which makes the process uneconomical.
The most familiar prior art process is the Lurgi process developed in Germany in the 1930's. Similar processes include the Winkler and Koppers-Totzek processes.
The Lurgi gasifier includes a water-jacketed furnace having a metal grate on which the input coal rests. Oxygen or air is introduced below the grate in sufficient quantity to cause combustion of approximately 20% of the coal. The combustion provides the necessary heat to gasify the remaining coal in the presence of steam. Ash and char fall through the grate and are taken out at the bottom. The obvious disadvantage of the Lurgi process is that approximately one-fifth or more of the coal is oxidized in order to sustain the operating temperature within the furnace. This oxidation results in a substantial dilution of the raw gas by carbon dioxide. If air is injected into the furnace for combustion instead of costly pure oxygen, the raw gas is further diluted by nitrogen. The control of the Lurgi process is, at best, complex. Assume that an input coal is introduced with a high moisture content. This will require additional heat for gasification. Since the heating process is combustion in nature, additional heat means additional oxygen to burn additional coal. Since the coal-to-steam ratio is disturbed by burning more coal, the steam flow would also have to be adjusted. The net effect is an adjustment of both oxygen and steam flows. The incremental heat is necessarily limited by the fusion temperature of the coal and ash and the melting point of the metal grate. Too high a temperature will fuse the char and ash and may also melt the metal grate. This limitation forces the Lurgi process to use coal of 9,000 Btu per pound or less, for fear of melting the grate or fusing the coal ash and plugging the grate. It is obvious, therefore, that to control the Lurgi process it is necessary to adjust all of the input parameters: oxygen, steam and coal.
The Atgas process developed by Applied Technology Corporation represents a significant departure from the Lurgi process. According to the Atgas process, a pool of molten iron is provided at the base of the gasifier furnace. The iron is initially melted by natural gas burners. After the iron is melted, coal which has been ground to about one-eighth inch is injected with a lance into the molten iron bath and, at the same time, oxygen is blown into the bath. A portion of the carbon in the coal and the oxygen reacts to form carbon dioxide and to produce the heat necessary to sustain the process. The heat of the process causes the coal volatiles to be immediately released. The remaining carbon dissolves into the bath where it reacts with steam to yield essentially carbon monoxide and hydrogen gas. The process is conducted at approximately 2,500 degrees Fahrenheit. The Atgas process has as a principal advantage its ability to significantly reduce the hydrogen sulfide that enters the raw gas. A large percentage of the sulphur in the coal dissolves in the iron and then diffuses to a molten slag layer floating on the top of the iron. However, the drawback, which is common to Atgas, Lurgi, Koppers-Totzek, Winkler and all other known processes for converting both the volatiles and char of coal to gas, is that a large portion of the carbon must be burned within the furnace in order to supply the heat requirements for the process. As a result, the raw gas is diluted with large amounts of carbon dioxide having no heating value. If the process is not equipped with a costly supply of pure oxygen gas, the raw gas is further diluted by nitrogen from the air used for the combustion. Another problem with the Atgas process is that the injection of coal, steam and oxygen into the molten iron bath presents serious problems of material handling. Furthermore, the injection of steam is extremely dangerous since the presence of condensation in the steam injection line could lead to a serious explosion. The Atgas, Lurgi and other processes for coal gasification for pipeline gas are described in "Evaluation of Coal - Gasification Technology; Part 1, Pipeline-Quality Gas" prepared for the Office of Coal Research, Department of the Interior (October 1973).
Garrett Research and Development Company, Inc., of La Verne, California, has proposed a flash pyrolysis technique for partially gasifying coal during a very short residence time. Coal is rapidly heated by combustion in an oxygen-deficient chamber and the volatiles are stripped off to produce a hydrocarbon-rich gas. The Garrett process does not gasify the fixed carbon in the coal, i.e., only the volatile matter is released by heating and the remaining char is recovered to be utilized as a solid fuel for electrical power generation. As in all other known coal gasification processes, the process heat for the Garrett process is supplied by combustion of the volatile gases with pure oxygen or air. The Garrett process is termed a "pyrolysis" process since the volatiles are released in a chamber deficient of oxygen to completely burn the volatile gases. It should be noted that the Garrett process provides only a partial gasification of coal in that only the volatiles are released. The residence time of the coal is only two seconds or less, thus making it impossible to gasify the fixed carbon in the coal. The char, comprised of the fixed carbon plus ash, remains after the partial gasification and is adapted to be conveyed to a nearby electrical power generator as a solid cake fuel. The Garrett process is described in the June 1974 issue of "Chemical Engineering Progress" at pages 72-75 and in U.S. Pat. Nos. 3,698,882 and 3,736,233.
The prior art has taught the use of electric arc technology as well as plasma arc technology for gasifying products. The earliest known electric arc process for gasifying coal is disclosed in U.S. Pat. Nos. 1,249,151 and 1,282,445 to B. F. McKee. In Research and Development Report 34 entitled "Arc-Coal Process Development" submitted by Avco Corporation, Systems Division, Lowell, Massachusetts 01851, to the Department of Interior, Office of Coal Research, the devolatilization and partial gasification of coal is accomplished using a heat source provided by a rotating electric arc. Plasma arc technology has been used in the conversion of municipal and industrial refuse into useful solid, liquid and gaseous products and having as a primary object the reduction in physical weight and volume of the refuse. U.S. Pat. No. 3,779,182 teaches a refuse conversion system having a furnace chamber into which is introduced a volume of unsegregated refuse. The refuse is maintained in contact with the arc of a plasma generator so as to reduce the refuse to molten liquid and gaseous products by pyrolysis in the absence of a reactant gas in the reduction chamber. The refuse is effectively stacked and assumes various levels in a tiered array. Other prior art teaches gasifying tiered layers of coal.
U.S. Pat. No. 3,422,206 discloses an electric furnace having three side-mounted arc devices for melting discrete batches of metal. This patent recognizes that a specific angular relationship between the torches will impart angular momentum to the bath surface to produce a "stirring" effect. Neither the nature of this "stirring" nor its impact on the overall process is disclosed. A similar furnace construction using oxy-fuel burners is described in U.S. Pat. No. 3,459,867.
A process for rapidly decomposing coal using an electric arc as the heat source is described in U.S. Pat. No. 3,384,467. The coal is introduced at the base of the furnace by a screw feeder, and the coal itself carries the arc current. The coal is devolatilized in approximately three seconds with a coal energy absorption rate of approximately 600 Btu/lb-sec. The gas products of this process represent only 15 percent of the weight of the initial coal input. No reactant is introduced into the furnace, and the fixed carbon is not gasified.
Also to be noted is that various prior art processes require the vessel or the heat source to be rotated. Also, the coal or other matter being gasified is often required to be forced up vertically through a bed of material being gasified which requires heavy and sometimes complex feeds.
A study of the prior art indicates that there is an acute need for a reliable and efficient system and method for gasifying carbonaceous matter, especially coal, having the following characteristics: (1) The heating process is decoupled from the gasification process; (2) The system is adapted to release the volatiles and gasify the fixed carbon of coal on a continuous basis in one vessel; (3) The endothermic heat is supplied by efficient electrical means, vis: long arc column plasma torches; (4) Neither the plasma torch nor its vessel is rotated during gasification: (5) The furnace temperature can be relatively high and can be controlled principally by a single control, i.e., torch power; (6) The char has maximum surface area of exposure to accelerate the gasification reactions; (7) The char can be maintained in motion during gasification; (8) Any grade or type of coal may be devolatilized and gasified in the same chamber without extensive pretreatment such as washing, drying, fines removal or agglomeration; and (9) Gravity feed may be employed for the incoming coal.
The achieving of the foregoing characteristics in an integrated system and process thus becomes an object of this invention.